almost 9 years ago
Character Ages (Basic)
The character ages on Teen Wolf are notoriously difficult to pin down, as the show does not seem to place much emphasis on creating a coherent timeline.
Best guess is that Scott and Stiles are 17 in Season 1 and just starting their sophomore years. The ages are based off the drivers license status and the laws of California.
Stiles already has the legal right to drive in [[1x01 - Wolf Moon]]. California law states that at 15.5 years, a teenager can get a provisional permit, which requires that they be accompanied by a licensed driver at least 25 years old at all times. At 16, teens can apply for a provisional driver's license. This allows them to drive solo, except between 11 p.m. and 5 a.m. or when they have a passenger under the age of 20. At 17, the passenger age restriction is dropped, but the curfew remains. At 18, drivers get a regular license. 
If Jackson has indeed been captain of the lacrosse team for 3 years, as Lydia implies, then this would make him a junior instead of a sophomore and likely 1 year older than Scott and Stiles. If, however, the Cyclones aren't state champions for 3 years running only because Jackson was captain that whole time, then he could be a sophomore with the rest of them.
Jeff Davis has stated in a Q&A that the boys are starting their sophomore year.
Derek's age is unknown, even to Tyler Hoechlin. He thought he was supposed to be 19 in Season 1 but was quicky changed to 26, likely because Hoechlin doesn't look remotely 19. This would mean Derek was college-age when the Hale House fire happened. Assuming that year has passed since the show began, Derek was most likely 25 in Season 1 and 26 in Season 3.